1. Field of the Invention
The present invention relates to a two-dimensional or three-dimensional coordinate measuring machine for measuring the size and shape of a work placed on a table from the displacement of a measuring member which is moved in multidimensional directions and contacted with the work and, more particularly, to a coordinate measuring machine which needs no mechanism for interconnecting opposite lower ends of a bridge-shaped measuring member support body under the table. 2. Description of the Prior Art
The coordinate measuring machine for measuring the size and shape of a work has widely been used in various industrial fields owing to its high measuring accuracy and measuring efficiency. A conventional coordinate measuring machine comprises a table having an upper surface for placing a work thereon, a bridge-shaped measuring member support body extending over the table and capable of moving relative to the table, and a measuring member supported by the measuring member support body, in which the measuring member is moved in multidimensional directions and contacted with the work placed on the table, and the size and shape of the work is measured from the displacement of the measuring member.
FIG. 7 illustrates an example of a conventional three dimensional coordinate measuring machine having a fixed table 10 and a movable measuring member 12. This conventional three-dimensional coordinate measuring machine comprises a table 10 having an upper surface 10A for placing a work thereon, a bridge-shaped measuring member support body 14 having a bridge-shaped crossing over the table 10 and movable in a direction of Y-axis respective to the table 10, and a measuring member 12 supported on the measuring member support body 14.
The table 10 is mounted on a base 16.
The measuring member support body 14 comprises a left column 18, a right column 20, a crossmember 22 which is across the table 10 along X-axis and extends over the upper portions of columns 18 and 20, and X-axis slider 24 movably mounted on the crossmember 22 along the X-axis, and a spindle 26 movably mounted on the X-axis slider 24 along vertical direction of the upper surface 10A of the table 10, namely, along Z-axis. the measuring member 12 is attached to the lower end of the spindle 26. In FIG. 7, designated at 27 is a casing of the spindle 26.
In this type of three-dimensional coordinate measuring machine, the measuring member 12 is moved in a direction of the Y-axis by moving leg portions 19 and 21 of the measuring member support body 14 along a guide rail 28 fixed on the upper surface of the table 10. Namely, the right leg portion 21 (i.e., Y-axis slider) of the column 20 of the measuring member support body 14 is provided with bearings associated with the upper surface of the guide rail 28, for example, air bearings 30, and with bearings associated respectively with the opposite side surfaces of the guide rail 28, for example, air bearings 32 as shown in FIG. 8. Further, the left leg portion 19 of the column 18 of the measuring member support body 14 is provided with bearings associated with the upper surface 10A of the table 10, for example, air bearings 34 as shown in FIG. 8.
The measuring member 12 is moved in a direction of the X-axis by moving the X-axis slider 24 along the crossmember 22. Furthermore, the measuring member 12 is moved in a direction of the Z-axis by vertically moving the spindle 26 relative to the X-axis slider 24.
A main scale 36 of a Y-axis encoder 36 is fixed to the guide rail 28. The Y-axis encoder 36 detects the displacement of the measuring member 12 along the Y-axis with the movement of the measuring member support body 14 along the guide rail 28. A main scale 38 of an X-axis encoder 38 is fixed to the crossmember 22, and the displacement of the measuring member 12 along the X-axis with the movement of the X-axis slider 24 is detected by the X-axis encoder 38. The displacement of the measuring member 12 along the Z-axis by the vertical movement of the spindle 26 is detected by a Z-axis encoder 40 attached to the casing 27 (X-axis slider 24).
Thus, size and shape of the work fixedly placed on the upper surface 10A of the table 10 can be measured by moving the measuring member 12 in contact with the surface of the work in three dimensional directions.
In this type of three-dimensional coordinate measuring machine, the measuring member support body 14 is guided while moving along the Y-axis by the opposite side surfaces and the upper surface of the guide rail 28 provided on one end of the upper surface 10A of the table 10, and by the other end of the upper surface 10A of the table 10 as schematically shown in FIG. 8.
However, this conventional coordinate measuring machine has such problems as follows: (1) The provision of the guide rail 28 on the upper surface 10A of the table 10 inevitably reduces the effective area on the upper surface 10A for placing a work so that the work to be placed on the table 10 is limited in size. (2) The guide rail 28 is an obstacle to carrying in a work on the table 10, so, in some cases, the work must be tilted before mounting on the table 10. It leads to a hindrance to carrying the work in and out. (3) the provision of the guide rail 28 inevitably increases the height of the measuring member support body 14, hence the total height of the coordinate measuring machine. (4) The guide rail 28 provided on the upper surface 10A of the table 10 is apt to be soiled to make the smooth movement of the measuring member support body 14 difficult.
To solve those problems, UK Patent Application GB No. 2179452A was proposed.
One of the preferred embodiments according to the UK Patent Application is illustrated in FIG. 9. In contrast to the coordinate measuring machine shown in FIG. 7, in the embodiment a stone table 10 having a rectangular longitudinal section and a horizontal upper surface 10A is supported on the base 16 by supporting members 42 with a gap between the lower surface thereof and the upper surface of the base 16; the leg portions (i.e., Y-sliders) 19 and 21 of the measuring member support body 14 are provided with air bearings 30 which are opposite to the upper surface 10A of the table 10 and air bearings 32 which are opposite to the vertical side surfaces 10B and 10C of the table 10; and the leg portions 19 and 21 are interconnected by a connecting member 44 extending across and below the table 10, so that (1) any guide rail need not be provided on the upper surface 10A of the table 10, (2) the measuring member support body 14 is supported on the table 10 not to be inclined or caused to fall down by an external lateral force, and (3) the rigidity of the measuring member support body 14 is enhanced to prevent the expansion of the interval between the columns 18 and 20 of the measuring member support body 14 by air pressure applied to the air bearings.
In FIG. 9, denoted at 46 is a connecting member interconnecting the respective upper ends of the columns 18 and 20 of the measuring member support body 14.
In the coordinate measuring machine shown in FIG. 9, the measuring member support body 14 is guided while moving along the Y-axis by the horizontal upper surface 10A and opposite vertical side surfaces 10B and 10C of the table 10, as schematically shown in FIG. 10. Therefore, the side surfaces 10B and 10C having a wide span therebetween prevents the yaw of the measuring member support body 14.
FIG. 11 shows another embodiment according to the UK Patent Application GB No. 2179452A. This coordinate measuring machine comprises a table 10 having flat surfaces on a lower surface 10D extending in parallel to the upper surface 10A at the opposite ends thereof with respect to the X-axis, a guide rail 48 having two opposite vertical surfaces extending in the direction of the Y-axis is parallel to the Z-axis, attached to the lower surface 10D substantially at the middle thereof, the air bearings 30 provided on the leg portions 19 and 21 so as to be associated with the upper surface 10A of the table 10, air bearings 50 provided on the leg portions 19 and 21 opposite to the air bearings 30 so as to be associated with the flat surfaces formed on the lower surface 10D of the table 10, the air bearings 32 provided on the connecting member 44 interconnecting the leg portions 19 and 21 so as to be associated with the vertical surfaces of the guide rail 48, and the measuring member support body 14 movable relative to the table 10 along the Y-axis. Thus, the measurement error due to the tilt of the measuring member support body 14 is eliminated.
In this coordinate measuring machine shown in FIG. 11, as schematically shown in FIG. 12, the measuring member support body 14 is guided while moving along the Y-axis by the respective opposite ends of the upper surface 10A and lower surface 10D of the table 10, and by the opposite vertical surfaces of the guide rail 48, the rolling of the measuring member support body 14 is prevented by the respective opposite ends of the upper surface 10A and lower surface 10D of the table 10, and the yaw of the measuring member support body 14 is prevented by the opposite vertical surfaces of the guide rail 48 provided at the middle of the lower surface 10D of the table 10 having a narrow span between the opposite vertical surfaces.
However, in both of the foregoing prior arts, the rigidity of the measuring member support body 14 is enhanced to prevent the expansion of the interval between the columns 18 and 29 of the measuring member support body 14 due to air pressure applied to the air bearings, and the measuring member support body 14 is connected by the connecting member 44 extending across and under the table 10 to prevent the tilt of the columns 18 and 20. Accordingly, such a coordinate measuring machine requires additional parts. It leads to an increase in weight, difficulty in machining and assembling works of the parts, and higher price.
Furthermore, the coordinate measuring machine has a built-up construction, the table 10 must be solid to secure measuring accuracy. In contrast thereto, the measuring member support body 14 must be easy to operate and able to move accurately at a high speed. Therefore, the required material quality of the table 10 is usually different from that of measuring member support body 14, namely, the table 10 must be high in rigidity and low in thermal deformation, while the measuring member support body 14 must be lightweight furtheremore. Accordingly, in most cases, the table 10 is formed of stone, and the measuring member support body 14 is formed of cast aluminum and/or steel. However, problems arise in the coordinate measuring machine due to difference between those materials in thermal expansion coefficient. For example, the respective thermal expansion coefficients of aluminum, steel and stone are 22.times.10.sup.-6 m/deg, 11.times.10.sup.-6 m/deg and 8.times.10.sup.-6 m/deg. Consequently, the clearance between the measuring member support body 14 and the table 10 varies according to the difference between the table 10 and the long connecting member 44 in thermal expansion coefficient, due to the variation of the ambient temperature, namely, the clearance increases as the ambient temperature rises and decreases as the ambient temperature drops, and thereby the bearing clearance is caused to vary, particularly, when air bearings are employed, from an appropriate bearing clearance for highly accurate movement of the measuring member support body 14. Therefore, an appropriate mobility cannot be maintained.
Still further, the total height of the coordinate measuring machine is inevitably increased because a wide space must be secured between the lower surface of the table 10 and the upper surface of the base 16 to extend the heavy connecting member 44 across and under the table 10.